Solid-fueled and liquid-fueled "heterogeneous" nuclear reactors have already undergone extensive analysis and development. Gas-fueled nuclear reactor studies, however, have been restricted to either (a) homogeneous core systems, (b) large separate regions of fuel and moderator, or (c) concentric rings of fuel-moderator arrangements.
The arrangement of bundles of moderator cells or moderator/coolant channels arrayed in a fissionable gas or mixture of gases makes a truly heterogeneous nuclear reactor core. It is this core heterogeneity for a gas-fueled reactor which accounts for the novelty of this concept and leads to significant advantages, especially with respect to previous gas core systems. There are a number of advantages of the invention over solid fuel reactors. The fuel fabrication is essentially eliminated, which will reduce the fuel cycle cost. The requirements for engineering safeguards, especially the emergency core cooling system can be greatly reduced due to the inherent safety of an expanding gaseous fuel. Other advantages associated with gas-fueled reactors are high fuel utilization, simple fuel management, the possibility of continuous fission product removal, the possibility of continuous fuel recharge, high thermal efficiency and a reduction in nuclear weapon proliferation probability. In addition, one of the differentiating characteristics of this heterogeneous gas core reactor is its peculiar unit cell, in which the moderator is inside and the fuel is outside. Use of this arrangement results in less structural material and this leads to a better fuel and neutron economy, power distribution and reactor control. The present reactor is adaptable and flexible to conform to desired optimum conditions and to various specific applications.
A reactor system in accord with the invention further provides enhanced heat transfer characteristics. The present reactor makes use of minimal structural material for the moderator-containing tubes and is of compact design. The invention contemplates flexible and effective control possibilities. The capability to remove the fission generated energy from the heterogeneous gas core reactor core is notably better than that from a corresponding size homogeneous gase core reactor. This is not only because of the possibility of dual heat removal but also because of superior in-core heat transfer characteristics resulting from the core heterogeneity itself. This capability can be augmented by introducing vortices, swirl motion, buffer gases or various flow patterns as is deemed necessary.
Since the gaseous fuel is also a working fluid, a core coolant is not required to remove all the thermal energy from the core. Therefore, a core coolant, if used, can be operated at comparatively low pressures, and may be dispensed with entirely under some circumstances.
Due to the high operating temperature of the fuel, the overall efficiency of the present reactor can be as high as 40-50% depending on the design of the energy conversion system.
Unlike solid-fueled reactors, there are no significant problems associated with fuel rod swelling, hydriding and fuel melting in an HGCR. Materials related problems are, in this invention, associated primarily with the moderator channel rather than with the fuel. Therefore, this system can achieve much higher levels of fuel utilization than with other reactor concepts. In the present invention, almost all the fissile material can be utilized by blending depleted fuel with fresh fuel. The present reactor can be designed to attain a fuel utilization which is substantially greater than for other reactor concepts.
Due to the gaseous form of the fuel, its fabrication charge is eliminated. Also reprocessing costs will be greatly reduced and waste disposal simplified and reduced in magnitude.
Since recharging of the fuel can be accomplished simply by blending depleted fuel with fresh fuel, complicated fuel management programs such as exist for solid-fueled reactors are unnecessary.
Continuous fission product removal during gas cleanup operations decreases the initial inventory of the fuel and reduces the requirements of the total reactivity control elements. Also, the primary side contains a largely reduced inventory of radioactive fission products, to provide a beneficial impact on safety.
An HGCR with circulating gaseous fuel makes "continuous" or frequent fuel recharge possible. Fresh gaseous fuel can be added in a continuous or batch mode to the circulating fuel in the primary loop to keep the system functioning (with the required excess reactivity) without necessitating reactor shutdown.
In a heterogeneous gas core reactor, a sophisticated emergency core cooling system is unnecessary. The inherent safety of an expanding gaseous fuel can be engineered to replace many of the functions of other safeguard systems.
The high neutron flux in a heterogeneous gas core reactor permits induced nuclear transmutation of radioactive products or wastes. Induced nuclear transmutation of radioactive wastes is a process whereby long-lived radioactive wastes or products are converted into relatively harmless, short-lived or stable nuclides. This nuclear transmutation can significantly simplify the long-time, high level radioactive waste disposal problem.
The methods of energy conversion and/or utilization from this invention can range from the conventional steam cycles such as shown herein to, possibly, gas turbines or direct energy conversion schemes or to other applicable methods of utilization. Energy can be extracted or transferred from the hot core gas, from the in-core moderator/coolant materials, from the surrounding out-of-core moderator/reflector, from a blanket or blanket regions if so desired, from mechanical shaft power from pulsed versions of the basic concepts of the invention, or from nuclear pumped lasers.
The invention has great flexibility in that a variety of moderator and coolant materials are found to be quite acceptable.
The invention has great flexibility in that a wide variety of gaseous fuel mixtures consisting of both fissionable and non-fissionable gases are found to be effective.
A variety of reactor containment schemes are readily adapted for this invention.
A variety of core barrel and pressure vessel materials are found to be suitable for this invention.
The core (or multiple cores) are adapted to be surrounded by a suitable moderator/reflector material, a fissionable (fertile and/or fissile) blanket and shield regions if deemed necessary.
A central zone comprising, for example, moderator, void and actinide wastes, may be provided so that the reactor becomes an "annular core" reactor.
Fissionable material in other than gaseous form can be added to the core for the purpose of conversion, or breeding, or for augmenting the power producing capabilities.
A variety of methods are available in the reactor according to the invention for power flattening. These include conventional techniques along with some techniques which are quite different from those used in conventional reactors. For example, the spacing between coolant channels or cells can be varied in such a manner as to achieve power flattening. Power peaks occur at the highest temperature, and the peaks in the present reactor are flattened by providing moderator tubes less widely spaced in the high power regions of the core than in the lower power regions. This may be accomplished in this manner whether heat extraction exteriorly of the core is primarily from gaseous fuel medium or from circulating core coolant, such as circulating coolant gas circulated through graphite moderator material contained in the tubes, or from circulating light water moderator coolant. This method is very effective and does not suffer the harmful side effects often associated with conventional power flattening techniques. Peak-to-average flux ratios obtained for this invention are lower than for other power reactors.
The fact that the fuel is in a gaseous state and circulating adapts the reactor to a variety of control and burnup compensation methods which are not readily available with conventional reactors. Fuel and/or coolant flow rate changes, spectrum shifts, variations in fuel gas density and/or temperature, variations in gaseous fuel loop circulation time, fission product concentration changes and continuous or batch fuel blending procedures can all be used to minimize or eliminate the need for burnable poisons or control rods as a means for power level control and burnup compensation.
The invention has great flexibility in that it can readily function for a variety of geometries; these include but are not limited to cylindrical, spherical, slab and annular configurations.
The invention may readily be incorporated or combined with other energy generating schemes to form hybrid systems. An example is a fission/fusion device in which the fission region utilizes the principles embodied in this invention.
The gaseous fuel in the present invention lends itself to minimization of nuclear proliferation risk, and to accurate control and accounting of strategic material. The control and accounting can be performed on a batch or continuously monitored basis. The present reactor is an excellent burner of plutonium and can be used as a driver core for self-sustaining cycles. Additionally, by continuously separating selected fissile materials from the gas mixture, it is possible to maintain in-plant the plutonium and transuranium elements produced and thus enhance fuel utilization and minimize or eliminate out-of-plant plutonium handling. The invention contemplates that plutonium which is produced in the reactor will be recirculated until it is consumed.
Because of the efficient heat removal capabilities of the invention, it has a high power density, that is, the core size is small for any given megawatt capacity.
The reactor has potential not only for large electrical power generation but also for small (peaking) electricity generation. Nuclear reactors tend to be economically competitive only when the are incorporated into large capacity power systems. The fuel cycle cost of the present reactor and its less sophisticated engineered safeguards and auxiliary systems, make the present reactor competitive in smaller capacity power systems.
The reactor of this invention is able to better utilize neutronically the gaseous fuel in the core, particularly in the inner regions of the core, than other large gas core reactor concepts. The dispersion of moderator material through the core in bundles and the consequent reduction in selfshielding effects for the gaseous fuel are responsible for this improvement.
Because of the core power distribution, the efficient heat transfer and flow patterns, the invention is able to achieve better temperature distributions in the core and smaller temperature gradients.
The heterogeneous gas core reactor as described may be used for the burnup of radioactive waste materials by positioning rods of materials contained in tubular containers in the core or in the surrounding reflector or blanket region.